The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The near infrared wavelengths extend from 0.75 micrometers to 10 micrometers. The far infrared wavelengths cover the range from approximately 10 micrometers to 1 millimeter. The function of infrared detectors is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects dissipate thermal energy having characteristic wavelengths within the infrared spectrum. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. No single detector is uniformly efficient over the entire infrared frequency spectrum. Thus, detectors are selected in accordance with their sensitivity in the range of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detectors have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared frequency energy in one of several ways. Thermal detectors respond to infrared frequency energy by absorbing that energy, thereby causing an increase in temperature of the detecting material. The increased temperature in turn causes some other property of the material, such as resistivity, to change. By measuring this change, the infrared radiation is measured.
Photo-type detectors (e.g., photoconductive and photovoltaic detectors) absorb the infrared frequency energy directly into the electronic structure of the material, inducing an electronic transition which, in turn, leads to either a change in the electrical conductivity (photoconductors) or to the generation of an output voltage across the terminals of the detector (photovoltaic detectors). The precise change that is effected is a function of various factors including the particular detector material selected, the doping density of that material, and the detector area.
By the late 1800's, infrared detectors had been developed that could detect the heat from an animal at one quarter of a mile. The introduction of a focusing lens constructed of materials transparent to infrared frequency energy, as well as advances in semiconductor materials and highly sensitive electronic circuity have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements, the outputs of which are connected to sophisticated processing circuity. By rapidly analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and monitor sources of infrared radiation. Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared detection systems have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and the reliability and economical production of the detector array and accompanying circuitry.
The outputs of the detectors must undergo a series of processing steps in order to permit derivation of the desired information. The more fundamental processing steps include preamplification, tuned bandpass filtering, clutter and background rejection, multiplexing and fixed noise pattern suppression.
Limitations on the performance of contemporary infrared detectors arise due to the presence of noise intermixed with the signal generated by the detector. This is particularly true when noise is introduced into the signal prior to preamplification of the signal. In general, such noise may be a consequence of background conditions at the site of the detector, generated from within the detector itself, or generated as a consequence of the interconnected electronic circuitry. Unless eliminated from the detected signal, noise components establish the minimum level of detectivity obtainable from the detection circuit.
Cooling the detector to extremely low temperatures, i.e., cryogenic temperatures, is effective to reduce random electronic activity within the detector and therefore reduce some components of the noise spectrum. Filters and well-known electronic signal processing techniques are also effective to reduce background noise levels, permitting enhancement of the signal-to-noise ratio of the detector circuit.
It is necessary to isolate the detectors from their associated signal processing circuitry so as to maintain such cryogenic temperatures. As those skilled in the art will appreciate, signal processing circuitry, as well as any other electronic circuitry, would tend to increase the temperature of the infrared detectors. Thus, it is desirable to thermally isolate infrared detectors from any electronic circuitry.
One type of noise that is particularly significant when the detector operates in certain regions of the infrared frequency spectrum is commonly referred to as flicker noise, or 1/f noise. The term 1/f noise generally refers to noise in the semiconductor detector that is due to modulation of the detector conductivity. 1/f noise increases dramatically as the biasing voltage across the detector increases. Because 1/f noise can be the principal noise component at certain frequencies of operation, it is highly desirable that the detector bias voltage be reduced as much as possible while retaining high gain and low power levels in the detector circuit.
Although prior art circuits have been proposed that provide zero bias voltage across the infrared detector to reduce 1/f noise, those circuits suffer from one or more common deficiencies. One of those deficiencies concerns the ability of the circuit to achieve the desired operating point, i.e., the zero bias condition, and to maintain that desired operating point over time. Another deficiency concerns the ability of the circuit to achieve the desired operating point uniformly over a large number of inputs, despite differences in the threshold levels of the particular semiconductors incorporated into the circuit. Variations with regard to those threshold levels, as well as variations in the precise resistance of other components may change the operating conditions such that maximum reduction of noise levels is not consistently obtainable without persistent measurements and adjustments.
Another deficiency of contemporary zero biasing circuits relates to the intrinsic topology of the of the buffer circuits that are interconnected to the detectors. Ideally buffer circuits should operate at very small signal levels and therefore generate discernable output signals upon the application of a small signal upon the gate of the device. Thus, power consumption and power dissipation requirements should be minimized without sacrificing sensitivity to low level inputs. Some existing buffer circuits incorporate semiconductor devices such as bipolar transistors, which suffer from the requirement that too large a current must be applied to the base thereof in order to turn on the circuit. Thus, sensitivity is reduced in such buffer circuits. Other circuits that incorporate devices operating in a normally on condition, i.e., above threshold, may draw too much current when activated by a signal responsive to irradiation of the detector, thereby undesirably raising the bias voltage on the detector.
Contemporary zero biasing circuits typically function by first sensing a drift in the bias, which results in a non-zero bias, and then correcting the non-zero bias condition. Therefore, a non-zero bias condition exists momentarily prior to the correction. This results in a time variation of the detector bias and consequently generation of a 1/f noise component. Bias voltages are continuously applied to the detector element by signal processing circuitry and these bias voltages are continuously compensated for by the contemporary zero biasing circuits.
The electronic interface circuitry utilized in contemporary infrared focal plane arrays must transform extremely minute infrared detector signal current into usable voltages. The output of contemporary infrared detector elements is typically less than 0.01 picoamps. Thus, the process of converting the current output of the infrared detector elements into such a usable voltage is very susceptible to noise. The amount of susceptibility to noise is substantially dependent upon the input characteristics of the transimpedance amplifier (TIA) utilized. The TIA is also required to operate at extremely low power levels so that the infrared detectors can easily be cooled so as to facilitate their operation at cryogenic temperatures as discussed above.
The input impedance of the TIA must be as low as possible such that a substantial portion of the infrared detector current output flows into the TIA rather than being shunted across the infrared detector element's own impedance. Such construction thus maximizes the input circuits injection efficiency. Longer wavelength infrared detectors and detectors operated at warmer than normal temperatures further require particularly low input impedance TIA's.
In order to obtain good system sensitivity, both the noise level of the TIA and the noise level of the infrared detector must be minimized. As mentioned above, one of the major components of detector noise is 1/f noise, which is strongly dependent upon detector bias. The TIA input circuitry must thus maintain the infrared detector bias as close to 0 volts as possible. In many applications, bias voltages exceeding several hundred microvolts can cause unacceptably high 1/f noise.
Furthermore, coupling of the output signal from the comparatively cold cryogenic focal plane to a comparatively warmer area wherein signal processing circuitry is disposed requires a low noise, signal preserving path which maintains the required high degree of thermal isolation. The need for a low noise, signal preserving path and a high degree of thermal isolation present conflicting requirements. The long path lengths necessary to improve thermal isolation are in contradiction to the short path lengths which are substantially less susceptible to noise and distortion.
Typical approaches to resolving this conflict include converting the analog detector signal into a digital signal utilizing an analog-to-digital converter followed by a fiber optic interface for communicating the detector output signal to the warmer signal processing circuitry. The optical fiber maintains the required thermal isolation while substantially reducing susceptibility to electrical interference. The penalty is high-powered dissipation and quantizing noise associated with the analog-to-digital converter and the high-power dissipation of the active fiber optic driver.
Examples of prior art input circuits which receive the output of an infrared detector element and transmit a signal representative thereof to signal processing circuitry are provided in U.S. Pat. Nos. 5,118,946 and 5,126,568. U.S. Pat. No. 5,118,946 discloses optical coupling of an infrared detector element to signal processing circuitry. U.S. Pat. No. 5,126,568 discloses magnetic coupling of an infrared detector element to signal processing circuitry. In both U.S. Pat. No. 5,118,946 and U.S. Pat. No. 5,126,568, various components, such as amplifiers, are located upon the focal plane so as to facilitate the transmission of a signal representative of the infrared detector element output to signal processing circuitry. However, such components inherently radiate heat, thereby undesirably tending to raise the temperature of the infrared detector elements and thus increasing 1/f noise.
As such, although the prior art has recognized to a limited extent the problems of thermal isolation and the generation of 1/f noise due to bias voltages generated by signal processing circuitry, the proposed solutions have to date been ineffective in providing a satisfactory remedy. In view of the shortcomings of the prior art, it is desirable to provide an input circuit which isolates the detector element from both heat and the bias voltage generated by signal processing circuitry so as to reduce 1/f noise.